Many current radio frequency applications are critical with regard to the stability of the signal path attenuation, the signal path phase length, and the signal path return loss. A component which is frequently found in the signal path, and one which is well known to be a major contributor to signal path instabilities, is the flexible transverse electromagnetic mode (TEM) transmission line, which is often subject to flexure during use. This flexure most often also applies torque forces to the transmission line, in that one end of the line is displaced rotationally from the opposite end of the line, which causes twisting of the transmission line. Further, since such transmission lines are often handled during use, they are sometimes subject to accidental crushing.
TEM transmission lines are of coaxial geometry. They consist of a center conductor concentrically surrounded by a dielectric medium, one or more tubular outer conductors, and an insulating outer jacket. The line is terminated by two coaxial connectors which allow the line to be connected to equipment with mating counterpart connectors.
The combination of the coaxial geometry of the line and its physical restraint at both ends via the attached coaxial connectors dictates that when the line is bent, as during flexure, physical path lengths within the line must change. In particular, the path length of the tubular outer conductor must increase on the outside of the bend, and must decrease on the inside of the bend. This is due to a difference in bend radii for each path, said difference being equal to the cable diameter, and the connector restraint, which results in an extension force applied to the tubular outer conductor at the outside of the bend, and a compression force applied at the inside of the bend. To a lesser extent, the dielectric medium and the center conductor are similarly distorted. These path-length changes are magnified with decreasing bend radii, and, at some point, failure of the tubular outer conductor will occur due to the stresses involved, quite often damaging the dielectric medium as well.
Torque forces which are applied to the line twist the outer conductor, in effect altering its physical path length. If the twisting is severe enough, the diametrical relationship of the outer conductor to the center conductor is altered and/or the concentric relationship of the center conductor, dielectric medium, and tubular outer conductor is disturbed. If crushing forces are applied to the line, non-concentricity will result.
In general, even minor physical path-length changes, alterations of concentricity, changes in diametrical relationship, or distortions of any single element of the TEM transmission line will cause the electrical characteristics of phase length, attenuation, and return loss to change. This is of little or no consequence in most microwave applications where the TEM transmission line is bent for routing but is not flexed during use. In these cases, the change of electrical characteristics is usually slight. Further, systems which are critical to such slight changes are usually designed so that the results of such changes are negated via adjustment, and since the line remains fixed in position, the net change is zero.
A TEM transmission line which is subjected to flexure during use, however, presents a quite different problem. Since it is subjected to bending and torque in a nearly infinite number of radii, bend planes, compound bend planes, etc., changes of electrical performance are of a dynamic nature and not predictable in extent. In test equipment applications, in particular, this may present a severe problem. This equipment is set to a zero reference with the TEM transmission lines in a fixed position. When the cables are flexed during the movement necessary to connect them to the item under test, dynamic changes in electrical performance occur, to some degree shifting the reference from zero and introducing non-predictable errors in the measurements performed. This condition is commonly referred to as transmission line instability error.
It is well known in the art that the degress of instability increases with decreasing bend radius and with increasing torque forces. It is also known that the useful life of the transmission line decreases as the bend radius is decreased and the amount of twist (torque) is increased. There is, in fact, a bend radius and/or an angular displacement due to twisting that will permanently degrade or possibly destroy the electrical performance characteristics of any microwave coaxial transmission line. Crushing is, of course, catastrophic in nature.
Due to these considerations, it has been usual in applications which require flexure to attempt to limit the amount of transmission line instability, and extend the useable life, by specifying the allowable bend radius, torque forces, and crushing forces. In practice, however, such specifications are unenforceable. Strict adherence to said specifications becomes the exception rather than the rule, since even if conscious efforts are made to adhere to such specifications, a single mistake (perhaps not even noticed) can physically alter the transmission line to the extent that its stability becomes considerably less than that required, and the useful life of the transmission line is shortened or terminated. This is a result of the inherent physical characteristics of most transmission lines, which allow them to be easily bent to a radius tighter than specified, to be twisted (torqued) an undesirable amount, or to be easily crushed. Even unusual provision for care cannot preclude this occurrence. Attempts to rectify this problem have previously resulted in either very springy, or bendable but not flexible, lines which can still be destroyed with relative ease.
The present invention corrects this situation by employing an external mechanical means for limiting the allowable degree of physical manipulation that the transmission line can experience. This is accomplished by restricting the bend radius to a minimum value, said value being dictated by the attributes of the microwave coaxial transmission line used and the requirements of the application, minimizing the torque forces which are applied to the microwave coaxial transmission line, not allowing it to be excessively twisted, and providing crush resistance to the transmission line. As a result, consistent electrical stability and longer useable life are provided. Further, the present invention retains a high degree of flexibility when bent to any radius larger than the minimum restricted radius.